This invention relates to an improved method and apparatus for applying a multicomponent coating composition to a substrate to refinish the same. In particular, the present method involves metering and proportioning a controlled ratio of the components of the system, prior to mixing. The ratioed components are then sprayed immediately onto the substrate. This invention is especially useful in the automotive refinish industry for such coatings as clearcoats, basecoats, and primers.
There presently exists a variety of systems for proportioning components and delivering them to atomizers or dispensing equipment in proper ratios. For example, such systems are disclosed in U.S. Pat. No. 4,966,306; U.S. Pat. No. 4,953,754; U.S. Pat. No. 4,529,000; U.S. Pat. No. 3,776,252; U.S. Pat. No. 3,672,389; and U.S. Pat. No. 3,530,873. Devices for proportioning coating compositions are marketed and have found wide use in original equipment manufacturing (OEM) of automobiles and other industrial equipment. However, such devices are not generally used in automotive refinish or body shops, where only one vehicle at a time is normally painted or finished.
The term "automotive refinish" refers to the application of a finish to an automobile subsequent to the original manufacturing process. In the OEM factory, the metal body of an automobile is typically coated or painted in an assembly line process, permitting the use of coating compositions on a large scale which are cured at elevated temperatures, typically as high as 150.degree.-160.degree. C. However, once the car has been fitted with plastic bumpers, rubber tires, and the like, it is no longer feasible to cure finishes at high temperatures. In the automotive refinish context, coatings normally are cured at ambient temperatures, although cure time may be accelerated by heating to temperatures up to 80.degree. C.
In refinish applications, the coating material being applied to a substrate as a finish is typically the product of a multipackage system that has been mixed manually prior to use. In a typical two component system, the first package is composed primarily of an acrylic interpolymer containing crosslinking monomer units. The second package is composed of the crosslinking agent required to react with the polymer in the first package. The proper volumetric mix ratio of the components is determined by the proper stoichiometric ratios of the reactive parts of the components needed for the crosslinking reaction to take place. Either package may also contain catalysts for promoting and initiating the crosslinking reactions, as well as additives, reducers, and pot life extenders. In some cases, more than two packages or components may be involved, for example, a catalyst may be present in a third component.
Conventional refinish methods, for applying a coating composition to a substrate, have been limited in several significant respects. Typically, the components of the coating composition are mixed manually. Once mixed, the composition must be used within a certain time frame. The potlife is defined as the time during which the mixture is suitable for spraying. More specifically, it is the point at which the applicator can perceive a discernable difference in the ease of handling due to an increase in the viscosity of the mixed components. This time frame is subjective and can vary, depending on the particular reaction involved, from an increase of several seconds to tens of seconds, according to a Zahn #2 measurement of viscosity. The Zahn cup is a fixed volume cup with specific orifice size. The amount of time it takes for a particular mixture to flow through the orifice is indicative of the viscosity of the mixture. Although this pot life characterization is to some extent subjective and dependent on the chemistry and applicator, in general, for the present purposes, pot life will be defined as a doubling of the viscosity (centipoise). For conventional applications, the paint or finish material must have a pot life of at least about 2-3 hours in order to give the user ample time to effect the refinishing task. Such a constraint limits the formulating latitude of the coating formulator. In particular, new high solids and/or low VOC (i.e., low content of volatile organic compounds) compositions have been difficult to develop because of problems of short pot life with such compositions. Such high solids compositions for coatings tend to exhibit a shorter pot life and rapid increases in viscosity, due to the higher concentration of reactants. This is hown in FIG. 4. This can present problems in applying them to substrates. On the other hand, if stabilizers or extenders are added to the formulation of the finish composition to increase the pot life, then the film drying and curing time is extended. This will increase the length of time needed to complete a job, thereby decreasing the productivity of the refinishing task. In addition, when the finish of an automobile is still wet, it is more susceptible to the introduction of defects, for example, caused by either accidental rubbing or air-bore contamination such as dust and dirt.
In view of the above, there is a need for an improved method of applying a refinish coating composition to an automobile or the like. It would be particularly desirable to solve the problems or difficulties associated with the formulation and spraying of high solids, low VOC coating compositions.